Hydroprocessing can include processes which convert hydrocarbons in the presence of hydroprocessing catalyst and hydrogen to more valuable products.
Hydrocracking is a hydroprocessing process in which hydrocarbons crack in the presence of hydrogen and hydrocracking catalyst to lower molecular weight hydrocarbons. Depending on the desired output, a hydrocracking unit may contain one or more beds of the same or different catalyst. Hydrocracking can be performed with one or two hydrocracking reactor stages. In single stage hydrocracking, only a single hydrocracking reactor stage is used. Unconverted oil may be recycled from the product fractionation column back to the hydrocracking reactor stage. In two-stage hydrocracking, unconverted oil is fed from the product fractionation column to the second hydrocracking reactor stage. Slurry hydrocracking is a slurried catalytic process used to crack residue feeds to gas oils and fuels.
Due to environmental concerns and newly enacted rules and regulations, saleable fuels must meet lower and lower limits on contaminates, such as sulfur and nitrogen. New regulations require essentially complete removal of sulfur from diesel. For example, the ultra-low sulfur diesel (ULSD) requirement is typically less than about 10 wppm sulfur.
Hydrotreating is a hydroprocessing process used to remove heteroatoms such as sulfur and nitrogen from hydrocarbon streams to meet fuel specifications and to saturate olefinic compounds. Hydrotreating can be performed at high or low pressures, but is typically operated at lower pressure than hydrocracking.
In slurry hydrocracking, a three-phase mixture of heavy liquid oil feed cracks in the presence of gaseous hydrogen over solid catalyst to produce lighter products under pressure at elevated temperature. Many of the products from slurry hydrocracking may require further upgrading.
A hydroprocessing recovery section typically includes a series of separators in a separation section to separate gases from the liquid materials and cool and depressurize liquid streams to prepare them for fractionation into products. Hydrogen gas is recovered for recycle to the hydroprocessing unit. A stripper for stripping hydroprocessed effluent with a stripping medium such as steam is used to remove unwanted hydrogen sulfide from liquid streams before product fractionation.
In hydroprocessing recovery, liquid streams from a hot separator and a cold separator are let down in pressure prior to being routed to the fractionation section and the gases evolved are separated in the hot flash drum and a cold flash drum, respectively. The evolved gases are rich in hydrogen and these streams are often routed to hydrogen recovery facilities. In the absence of the flash drums the hydrogen solution loss contained in the liquid from the high pressure separators would otherwise be lost in the fractionation section off-gas. By keeping the pressure of the hot and cold flash drums high, recovered hydrogen streams can be sent directly to the makeup gas header without further need of compression. However, higher pressure forces more hydrogen into liquid solution, causing more loss of hydrogen in the fractionation off-gas which is typically sent to the fuel gas header.
Efficient use of hydrogen is critical to the economics of a hydroprocessing unit. A significant amount of hydrogen is lost in the hot flash liquid stream that is sent to the stripper column. This is called solution loss, and it represents a potential sizable loss of a valuable resource to the fractionation section where it ends up in the fuel gas. Methods have been disclosed to recover the hydrogen in the stripper off gas stream but requires compression to be able to route this stream to the cold flash drum off gas stream in which it can be routed to hydrogen recovery. Compression of the stripper off gas is expensive in both capital and operation.
There is a continuing need, therefore, for improved methods of recovering hydrogen gas from hydroprocessed effluents.